The field of the invention relates to a belt used for extracting water from a web of material, and more particularly from a fibrous web formed in a papermaking machine.
During the papermaking process, a web is formed by depositing a fibrous slurry on a forming wire. A large amount of water is drained from the slurry during this process, after which the newly-formed web proceeds to a press section. The press section includes a series of press nips. The web finally proceeds to a drying section including heated dryer drums where the water content is reduced to a desirable level.
In view of the high cost of energy, it is desirable to remove as much water as possible from the web prior to its entering the drying section. The dryer drums in this section are often heated by steam and costs can be substantial if a large amount of water needs to be removed.
The use of the extended nip press has been found to be advantageous over the use of nips formed by pairs of adjacent rollers. By extending the time the web is subjected to pressure in the nip, a greater amount of water can be removed. This fact has been recognized by those skilled in the art, and several patents have been granted in the area. These patents include Re. 30,268, 4,201,624, 4,229,253 and 4,229,254.
In using the extended nip press to dewater a fibrous web, the web has typically been sandwiched between two moisture-absorbing felts and a belt. The felts are trained around a cylindrical press roll with the web between them while the belt is arranged for applying pressure to the felts and roll. A pressure shoe exerts pressure on the belt in the press area and the shoe and belt are lubricated with a hydraulic oil.
A problem has been encountered during the dewatering of webs in extended nips. It has been found that a bulge develops in the belt ahead of the nip which in turn results in delamination. This problem is recognized in the U.S. Pat. Nos. 4,229,253 and 4,229,254 and certain belt constructions are suggested for overcoming the problem.